PITTSBURGH – President Barack Obama said today his administration has pulled the economy back from the brink and is determined to reinvigorate America’s middle class.

Shoring up a key part of his political base, Obama told a convention of the AFL-CIO that the administration is a fierce supporter of workers’ rights and defender of organized labor. He also challenged members of the country’s largest labor federation to stand with him and push ahead on a proposed overhaul of the nation’s health care system.

“One of the fundamental reasons I ran for president was to stand up for working families,” Obama said on a day designed to appeal to politically active union members.

“When our middle class succeeds, that’s when the United States of America succeeds. That’s what we’re fighting for,” he said.

In a message tailored to workers, Obama praised organized labor figures gathered in Pittsburgh for their role in creating a middle class and for propelling the economy forward during last century. He said the same groups must help push the economy move ahead now.

“I know too many people are still looking for work, worried they’ll be the next one to be let go,” a somber Obama said, nodding to a job market that has shed hundreds of thousands of jobs since he took office.

Earlier in the day, he toured a General Motors plant in Lordstown, Ohio, and claimed credit for an improving economy. He told an audience near Youngstown that the economy is on its way back from the brink because of his policies.

Pledging to stand firm with autoworkers in northeast Ohio, Obama told factory workers that his administration had no choice but to intervene and prevent the collapse of automakers. He told GM employees that their company has retooled itself and is heading back to a solid business, in part, because of its work force.

“Your survival and the success of our economy depended on making sure that we got the U.S. auto industry back on its feet,” Obama said, standing near a production line where compact Chevrolet Cobalts are produced.

He said those small cars were among the most popular under his temporary Cash for Clunkers program that offered drivers up to $4,500 to buy more fuel-efficient automobiles. General Motors has increased production of the compact auto and rehired laid-off workers to restore a second shift here.

“Because of the steps we have taken, this plant is about to shift into high gear,” Obama said. “A hundred and 50 of your co-workers came back to work yesterday. More than a thousand will be coming back to work in less than three weeks as the production of the Cobalt ramps up.”

At the same time, Obama acknowledged that recovery would not be simple or swift.

“Over the years, we’ve seen factories close. You’ve seen friends, neighbors and relatives laid off.,” Obama said. “Your daughters and sons have had to move away in search of jobs and opportunity. I know it was painful around here earlier this year when three shifts at this plant were cut down to one.”

But he said the nation cannot allow towns like this one – a major employer in a region that has seen steel furnaces go cold as globalization moved jobs abroad – to crack under the economic pressure.

“There are some who see this pain and suggest that somehow it’s inevitable, that the only way for America to get ahead is for communities like yours to be left behind,” Obama said. “But I know better. We know better. We know that our success as a nation depends on the success of communities just like this one.”

He also played to a populist theme, key to this hard-hit part of America.

“As long as you’ve still got an ounce of fight left in you, I’ll got a ton of fight left in me,” Obama said. “I’ve said it before: I’m skinny, but I’m tough.”

The events were designed to be heavy on working-class appeal in hopes of boosting the White House’s credentials with the segment of the population that played so large a role in his election and is so crucial to his economic agenda.

Obama wraps up his daylong trip at a fundraiser for Pennsylvania Sen. Arlen Specter in Philadelphia. He is expected to tell donors that Specter, who earlier this year switched from the GOP to the Democratic Party, is crucial for pushing the White House agenda through.